System and Method for Producing Ethanol and Biogas

Abstract

A system and process for producing ethanol and biogas. An incoming feedstock and water is directed to a preparation unit that frees sugar from the feedstock. The feedstock is directed to a fermenter that ferments the sugar to produce a beer that includes ethanol and feedstock. The beer is directed to a distillation unit which separates the ethanol from the feedstock and produces ethanol and whole stillage. The stillage derived is direct to an anaerobic membrane bioreactor unit. The stillage is subjected to anaerobic digestion in the anaerobic digester and this produces mixed liquor that includes suspended solids. Mixed liquor including the suspended solids is directed to the membrane separation unit which filters the suspended solids and produces a concentrated reject (retentate) stream and a backset permeate stream. The backset permeate stream is then mixed with the incoming feedstock.

1 . A method of producing ethanol and biogas comprising:
 (a) directing an incoming feedstock to a pretreatment area and pretreating the feedstock to free sugar from the feedstock;   (b) fermenting the sugar produced from the feedstock in a fermenter to produce beer which includes the feedstock and ethanol;   (c) distilling the beer to produce ethanol and whole stillage which includes liquid, dissolved solids and suspended solids;   (d) directing at least a portion of the whole stillage or a portion of thin stillage produced from the whole stillage to an anaerobic membrane bioreactor having an anaerobic digester and a membrane separation unit and:
 (1) in the anaerobic digester anaerobically digesting solids in the stillage and producing biogas and a mixed liquor; 
 (2) directing the mixed liquor from the anaerobic digester to the membrane separation unit and removing suspended solids from the mixed liquor and in the process producing a concentrated reject stream and a backset permeate that is substantially free of suspended solids; 
   (e) recycling at least a portion of the backset permeate and mixing the backset permeate with the incoming feedstock and wherein the mixture of the backset permeate and the incoming feedstock is fermented in the fermenter;   (f) stratifying the mixed liquor in the anaerobic digester by forming a first lower mixed liquor zone where the mixed liquor in the first lower mixed liquor zone includes a relatively high concentration of solids, and forming a second mixed liquor zone above the first lower mixed liquor zone where the mixed liquor in the second zone includes a solids concentration substantially less than the concentration of solids in the first lower mixed liquor zone; and   (q) directing mixed liquor from the second mixed liquor zone in the anaerobic digester to the membrane separation unit where the mixed liquor is separated into the concentrated reject stream and the backset permeate that is relatively free of suspended solids.   
     
     
         2 . (canceled) 
     
     
         3 . The method of  claim 1  including separating the whole stillage into the thin stillage and wet cake, and directing the thin stillage to the anaerobic digester and anaerobically digesting solids in the thin stillage and producing the biogas and the mixed liquor. 
     
     
         4 . The method of  claim 1  including directing the whole stillage to a centrifuge and separating thin stillage from the whole stillage, and directing at least a portion of the thin stillage to the anaerobic digester and anaerobically digesting solids in the thin stillage to produce the biogas and mixed liquor. 
     
     
         5 . The method of  claim 1  further including directing the whole stillage to a stillage separator and separating thin stillage from the whole stillage, and splitting the thin stillage into at least two streams, and directing one stream of the thin stillage to the anaerobic membrane bioreactor and anaerobically digesting solids in the thin stillage and producing the biogas and the mixed liquor, and directing the other thin stillage stream to an evaporator and producing dry distiller's grain (DDG). 
     
     
         6 . (canceled) 
     
     
         7 . The method of  claim 1  including recycling at least a portion of the concentrated reject stream to the anaerobic digester and mixing the concentrated reject stream with the mixed liquor in the anaerobic digester; and directing the mixed liquor and solids from the first lower mixed liquor zone to a solids separator and separating the mixed liquor and solids into a heavier solids stream and a lighter solids stream containing biomass. 
     
     
         8 . The method of  claim 7  including recycling at least a portion of the lighter solids stream containing biomass to the anaerobic digester and mixing the lighter solids stream with the mixed liquor in the anaerobic digester. 
     
     
         9 . The method of  claim 1  including providing a mixing action in the first lower mixed liquor zone and mixing the mixed liquor and solids therein; and maintaining the mixed liquor in the second zone in an unmixed state or in a state where the mixing action in the second zone is substantially less than the mixing action in the first lower mixed liquor zone. 
     
     
         10 . The method of  claim 1  wherein the method includes forming a third mixed liquor zone over the second mixed liquor zone; and wherein both the first and third mixed liquor zones are mixed with relatively heavy solids residing in the first mixed liquor zone and relatively light solids residing in the third mixed liquor zone. 
     
     
         11 . The method of  claim 7  wherein the solids separator includes a hydrocyclone. 
     
     
         12 . The method of  claim 7  wherein during certain time intervals both the membrane separation unit and the solids separator are operated simultaneously and wherein there is one mixed liquor flow from the anaerobic digester to the membrane separation unit and another mixed liquor flow from the anaerobic digester to the solids separator, and wherein the two flows are independent of each other. 
     
     
         13 . The method of  claim 1  wherein the first lower mixed liquor zone is mixed with mechanical mixers; and wherein the second mixed liquor zone above the first lower mixed liquor zone is unmixed. 
     
     
         14 - 19 . (canceled) 
     
     
         20 . The ethanol plant of claim  15  wherein the membrane separation unit includes one or more nanofiltration or reverse osmosis membranes. 
     
     
         21 . (canceled) 
     
     
         22 . A method of producing ethanol and biogas comprising:
 (a) directing an incoming feedstock to a pretreatment area and pretreating the feedstock to free sugar from the feedstock;   (b) fermenting the sugar produced from the feedstock in a fermenter to produce beer which includes the feedstock and ethanol;   (c) distilling the beer to produce ethanol and whole stillage which includes liquid, dissolved solids and suspended solids;   (d) directing the whole stillage to an anaerobic membrane bioreactor having an anaerobic digester and a membrane separation unit and:
 (1) in the anaerobic digester anaerobically digesting solids in the whole stillage and producing biogas and a mixed liquor; 
 (2) directing the mixed liquor from the anaerobic digester to the membrane separation unit and removing suspended solids from the mixed liquor and in the process producing a concentrated reject stream and a backset permeate that is substantially free of suspended solids; and 
   (e) recycling at least a portion of the backset permeate and mixing the backset permeate with the incoming feedstock and wherein the mixture of the backset permeate and the incoming feedstock is fermented in the fermenter.   
     
     
         23 . The method of  claim 22  including:
 stratifying the mixed liquor in the anaerobic digester by forming a first lower mixed liquor zone where the mixed liquor in the first lower mixed liquor zone includes a relatively high concentration of solids, and forming a second mixed liquor zone above the first lower mixed liquor zone where the mixed liquor in the second zone includes a solids concentration substantially less than the concentration of solids in the first lower mixed liquor zone; and 
 directing mixed liquor from the second mixed liquor zone in the anaerobic digester to the membrane separation unit where the mixed liquor is separated into the concentrated reject stream and the backset permeate that is relatively free of suspended solids. 
 
     
     
         24 . The method of  claim 23  including recycling at least a portion of the concentrated reject stream to the anaerobic digester and mixing the concentrated reject stream with the mixed liquor in the anaerobic digester; and directing the mixed liquor and solids from the first lower mixed liquor zone to a solids separator and separating the mixed liquor and solids into a heavier solids stream and a lighter solids stream containing biomass. 
     
     
         25 . The method of  claim 24  including recycling at least a portion of the lighter solids stream containing biomass to the anaerobic digester and mixing the lighter solids stream with the mixed liquor in the anaerobic digester. 
     
     
         26 . The method of  claim 22  wherein the whole stillage produced by distilling the beer is not subject to treatment in a solids separator prior to being directed to the anaerobic membrane bioreactor.